In an engine, when gas mixture is introduced to a combustion chamber during running, some unburned gas mixture leaks into the crankcase through the gap between the piston and the cylinder. This leaked gas is called blow-by gas, and it is prohibited by law to discharge this blow-by gas as is into the atmosphere as exhaust gas. For this reason, blow-by gas is returned to the intake port via a PCV (Positive Crankcase Ventilation) passage, reintroduced into the combustion chamber along with new gas mixture, and burned in the combustion chamber.
Blow-by gas contains lubricating oil such as engine oil in the form of an oil mist, and it is not preferable for blow-by gas containing oil mist to be returned to the intake port since oil will stick to the PCV passage and the periphery of the intake port. In view of this, an oil separator is provided inside the cylinder head cover or within the PCV passage as a means for collecting the oil mist in blow-by gas.
Patent Document 1 discloses an oil separator that employs multiple cyclones. In this oil separator, blow-by gas flowing in through a gas introducing opening is passed through a straightening chamber and introduced to multiple cyclones arranged in a line. Due to centrifugal force from the swirling flow produced inside the cyclones, the oil mist in the blow-by gas coagulates and becomes collected.